In the coated glass industry it is desirable to apply one or more thin layers of coating materials to one or both surfaces of the glass to provide desired characteristics to the finished coated glass product. For example, in the manufacturing of glass for a variety of applications, it is often desirable to apply infrared reflective coatings and/or other multilayer coating systems to provide desirable characteristics related to transmittance, emissivity, reflectivity, durability, color, photocatalysis and chemical resistance.
Presently, the controlled sputter deposition of coating materials is the most effective method of forming thin films on solid substrates. Compared with other thin-film formation methods such as vacuum evaporation, electroplating, electroless plating, and chemical vapor deposition, sputter deposition allows for a greater choice of film materials, higher purity and controlled composition of the film, greater adhesive strength and homogeneity of the film, and greater control of the film thickness.
Planar and cylindrical magnetrons are widely used for sputtering deposition of films on substrates. The substrate is positioned within a vacuum chamber containing at least one rotating cylindrical target or planar target that is comprised of or includes sputtering material on its outer surface. Sputtering occurs when energetic ionized particles impinge on the surface of a the target, causing the emission of particles, and typically erosion of the surface. Generally, the sputter coating process can continue until the cathode target is exhausted.
To achieve the sputtering process an electrical field is created between a cathode target and an anode in a vacuum chamber. Next, a gas, typically an inert gas such as argon, is introduced to the vacuum chamber. Electrons in the electrical field are accelerated and gain enough energy to ionize the gas atoms and create a glow discharge plasma. The resulting plasma is then attracted to the target, bombarding it and liberating target surface atoms. The charged plasma is maintained in a relatively narrowly defined area in front of the target by a magnetic field. As previously suggested, the target functions as the cathode in the sputtering system and a separate anode is provided in the chamber at a location spaced away from the target. Commonly, the chamber walls, a separate bar or another conductive member positioned away from the target functions as the anode.
To keep the plasma discharge active and local to the cathode target, it is necessary for the chamber to have a stable field enveloping the plasma and the cathode target. Typically, this is performed with permanent magnets, located behind or within the target, which confine electrons generated by a negative voltage close to the target surface thus keeping the plasma localized to the target surface. The magnets are usually of a permanent magnet type, arranged uniformly behind a planar target or arranged along a line within the rotating cylindrical target and held from rotation with the cylindrical target. The sputtering zone is created by the magnets extending along substantially the entire length and width of the planar sputtering target or extending along the length of the cylindrical sputtering target and only a small circumferential (radial) difference around it. Traditionally, the magnets are arranged so that the sputtering zone exists at the bottom of the planar or cylindrical target, facing a substrate being coated directly beneath.
Sputtering may be conducted in the presence of one or more gases. A first inert gas, such as argon, is commonly utilized for the production of the glow discharge plasma in a sputtering system. Additional gases may be supplied to the sputtering system if desired, such as one or more reactive gases. When conducted in the presence of a reactive gas such as oxygen or nitrogen, a reactive product of the coating material is deposited on the substrate, i.e., the coating layer is the product of the coating material and the reactive gas. For example, the introduction of a reactive gas such as oxygen or nitrogen to the chamber forms an oxide or nitride with the liberated target atoms.
A problem with many previous sputtering systems is that during the sputter coating process, liberated particles of coating material are deposited on non-substrate exposed surfaces within the vacuum chamber, such as the chamber walls and other mechanisms within the chamber including the one or more anodes. Over time, a layer of the target material or rejected target material will tend to accrete on the outer surface of the anode. The rate at which this accumulation occurs will vary depending on the power applied and the material of which the target is formed. While this accumulation is not desirable in the sputtering process for any targets, a coating of a conductive material generally does not unduly hamper performance of the sputtering system. However, if the material deposited on the anode is an insulator or dielectric material, this creates significant problems. For example, when a dielectric material such as zinc is sputtered in an oxidizing atmosphere to deposit zinc oxide, a coating of zinc oxide will be deposited on the anode, which reduces the effectiveness of the sputtering process. Since many coating materials or their reactive products are insulators, semiconductors, or are otherwise substantially electrically nonconductive materials (such as Al2O3, SiO2, Si3N4, TiO2), accumulation of these nonconductive materials on the anode causes a progressive slowing of the coating process, which ultimately results in the process shutting down. A nonconductive or dielectric material coating on the anode inhibits and eventually prevents charge carriers from flowing from the anode to the cathode, thus first reducing and eventually, in effect, stopping the sputtering process.
Similarly, undesirable coating of the gas distribution system positioned within the vacuum chamber may also cause problems related to adequate sputtering of coating material. For example, undesirable coating of the gas delivery members positioned in the vacuum chamber can cause insufficient reaction between the coating materials and the reactive gases, if a reactive gas is delivered through such members, or can inhibit the creation of plasma by limiting the amount of gas delivered to the chamber. Therefore, the result of coating build up on the gas distribution system may create overly metallic films or cause a slowing of the system due to insufficient plasma generation.
The sputtering zone also becomes difficult to control and maintain when the immediate area surrounding the cathode becomes coated with sputtering material. As this happens the charged plasma on the surface of the cathode is repulsed from the built up sputtering coating on the internal surfaces of the chamber due to their like polarity. As the repulsion of the plasma increases, the charged plasma expands outward away from the cathode in “search” of a conductive outlet. As the conductive areas of the chamber become randomly distributed and located further and further away from the plasma and the cathode, the uniformity of the plasma discharge deteriorates over time, thereby slowing the sputtering process. Eventually, the sputtering process needs to be stopped for cleaning. Furthermore, the non-uniformity of the plasma discharge typically reduces the quality of the thin film on the substrate.
Manufacturers will often try to stretch the chamber's productive operating period. This can be risky since near the end of a productive operating period the plasma discharge may be searching for non-coated areas in the chamber. To keep the plasma discharge active, manufacturers sometimes inject extra gas through the gas distribution system and into the chamber to create a non-insulated area. This extra gas can create a nucleation curtain or a clustering of the sputtered material. As a result, electrical current may be directed to flow through the nucleation curtain into the gas distribution system, seeking a path back to the power supply. This improper flow of electric current can cause the gas pipe to meltdown.
In addition to process downtime, the accumulation of electrically nonconductive or dielectric coatings on the anode of a sputter coating device may have other adverse effects on the coating process and the coating formed on the substrate. Nonuniformities may occur in the coating due to changes in the size of the conductive area of the anode. Moreover, the accumulation of electrically nonconductive material on the anode may contribute to arcing, thereby causing large pieces of coating material to drop off the cathode target or other coated positions within the chamber and onto the substrate. Furthermore, thick accumulation of a substantially nonconductive coating on the anode results in poor adhesion of the coating to the anode. As a result, flakes or pieces of material may fall off the anode and onto the substrate, thereby contaminating the coated surface. All of these adverse effects result in a nonuniform coating on the substrate to be coated.
These coating nonuniformities, as well as the accumulation of nonconductive material on the anode necessitate interrupting the coating process in order to clean or change the anode. This involves venting the chamber, careful cleaning, and reevacuating the chamber. Such nonuniformities and accumulation often occur at levels sufficient to require the user to frequently stop the process in order to reconditioning the anode. One conventional technique requires that production be shut down for as much as 6–8 minutes every hour so that the relative polarity of the cathode and anode can be reversed to sputter the accreted material off the anode surface. Other manual cleaning techniques, such as sandblasting the interior of the chamber may be utilized to reduce the accumulation of stray coating material. Cleaning the chamber can reduce production time and be very expensive.
Therefore, it would be beneficial to provide a sputtering apparatus comprised of a gas distribution system which contains one or more conducting anodic surfaces positioned in sufficient proximity to the cathodically charged target to maintain stable plasma distribution. Furthermore it would be desirable for a sputtering apparatus to provide the following features and/or benefits: a more constant and uniform depletion of the target; a more uniform coating process; a reduction in product variance by avoiding the problems associated with the accumulation of nonconductive material on anodic surfaces within the sputtering chamber; an improved sputtering system when the material being sputtered is a dielectric material; a number of spaced apart gas distribution nozzles as a series of separate anodes, thus permitting the size, shape, and position of the plasma to be easily controlled; a significant reduction in the slowing of the anode coating process over time; a significant reduction of the chamber becoming coated with insulating material; and, to reduce the need to stop the sputtering process for cleaning.